Free-machining aluminum alloys are well known in the art. These alloys typically include free-machining constituents such as lead, tin, indium and bismuth for improved machinability. These constituents form low melting point compounds which readily melt or soften due to the friction heat created during machining. Thus, material removal required for the manufacture of complex parts and components is easily facilitated.
During machining, free-machining alloys generate small chips or curls which are easily collected and do not interfere with the machining process. It is essential that these free-machining aluminum alloys form these small chips or curls for proper machining. Formation of long continuous strips or curls is totally unacceptable in machining since the curls or strips may wrap around the work piece or machining tool and disrupt the operation. Poor machinability also affects other machining operations since the operator must attend to a single machining operation and cannot effectively supervise a multiplicity of operations, as is commonly done in practice. AA6061 alloys are generally unacceptable for machining since they form these long continuous curls during machining.
U.S. Pat. Nos. 2,026,457 and 2,026,575 to Kempf et al. disclose free cutting aluminum alloys. Similarly, U.S. Pat. No. 4,005,243 to Baba et al. discloses a freely machinable aluminum alloy.
Other known machinable alloys include AA6262 and AA2011, 2012 and 2111.
While the prior art aluminum alloys provide adequate free machinability, they are not without drawbacks and/or disadvantages. For example, AA6262 contains lead and chips from machining this alloy represent a hazardous waste disposal problem, in addition to the possibility for exposure to fine lead particles during machining.
Prior art alloys containing bismuth, e.g., AA2011 and AA2111, can adversely affect the final mechanical properties of the machined part. Since bismuth has some affinity for magnesium, the bismuth in these alloys has a tendency to combine with the magnesium to prevent or reduce Mg.sub.2 Si formation potential for precipitation strengthening. Bismuth also has a poor affinity for tin, and alloys having these two components may not always form the desired low melting point compounds or structures for free machining.
As a solution to the problems identified above, the inventor has proposed free-machining aluminum alloys containing tin and indium as a means to eliminate both lead and bismuth as constituents in free-machining alloys. This alloy system is disclosed in U.S. Pat. No. 5,587,029, titled "Machineable Aluminum Alloys Containing In and Sn and Process for Producing the Same", issued Dec. 24, 1996 which is herein incorporated by reference in its entirety.
Although free-machining alloys containing the aforementioned indium and tin provide excellent machining properties, the levels of indium when combined with a high price for the source indium make the alloys somewhat unattractive from an economical standpoint.
Other free machining alloys suffer from inadequate Charpy V-notch impact strength. For example, AA6020 which has tin and high levels of silicon, instead of the lead and bismuth in A6262, exhibits poor impact properties. Alloys with low impact properties have a tendency to crack when deep drilled.
As such, a need has developed to provide an environmentally friendly free-machining alloy which does not have its final mechanical properties compromised by free-machining constituents therein, has good impact properties and which is even more economically attractive.
In some applications involving hot automotive brake fluid, alloys containing Sn have shown to be extremely poor in corrosion resistance. Hence, this new alloy is also aimed at removing Sn from alloys that require exposure to hot brake fluid.